1. Field of the Invention
The present invention relates to a pressure-casting method and apparatus for producing cast articles under a high pressure applied to a metal melt in a casting mold with a melt pressurizing plunger of a hydraulic cylinder exerting a stroke movement for compensating for shrinkage of the melt, particularly for producing cast articles of light metal alloy such as an aluminum alloy, magnesium alloy or the like.
2. Description of the Related Art
Such a pressure-casting method and apparatus are known, and are positively adopted for cast articles of light metal alloy requiring superior quantities regarding high strength and high pressure resistance, such as automotive vehicle parts. The melt pressure may be referred to as "a squeeze pressure", and the melt pressuring plunger may be called "a squeezing plunger". The squeezing plunger is a plunger different in some cases from the melt feeding or injecting plunger of a hydraulic cylinder, and in some other cases the injection plunger is used as the squeezing plunger after it has worked for feeding the melt.
With respect to the above pressure-casting method and apparatus, there is a known improved method or apparatus as disclosed in JP-A3-124358, wherein there is provided an oscillation transmitting rod other than the squeezing plunger that is disposed in a runner between a cavity and a gate in a casting mold to impart oscillation to the melt in the cavity through the gate by actuating the rod in a mechanical oscillation manner or a supersonic oscillation manner.
Further, JP-A3-71214 discloses another improved method or apparatus using an injection plunger as a squeezing plunger with a vibrator equipped to oscillate the melt.
Still further, there is a known improved method as disclosed in U.S. Pat. No. 5,119,866 corresponding to JP-A2-207960, wherein the melt in a mold cavity is pressurized with a squeezing plunger of a pressurizing hydraulic cylinder which is different from an injecting hydraulic cylinder by controlling the pressurizing hydraulic cylinder so that actual stroke movement of the squeezing plunger copies a desired curve or locus with respect to a desired oscillating stroke movement versus an elapse of time to thereby result in a melt in a mold cavity oscillating while the stroke compensates for the melt shrinkage.
JP-A3-7124, U.S. Pat. No. 5,119,866 and the present application are owned by the same applicant or assignee.
The above prior art methods or apparatus are advantages in improving a quality of cast articles, thanks to the oscillation of the melt. However, they are not yet satisfactory to obtain a target or desired quality, although hot tearing or cracking is reduced due to the melt shrinkage compensation under high squeezing pressure. In connection with this, the inventors have found that the cast articles produced by the prior art have in general a dominantly larger amount of columnar crystals generated with a lower amount of equiaxed crystals, and recognized for their various experiments that the equiaxed crystals contribute to a better quality of the cast articles regarding high strength and toughness behavior.